Modelling production-induced dynamic rupture of intra-graben faults and related earthquakes in the North German Basin

Abstract

Production-induced seismicity occurs in some gas fields of the North German Basin. Increasing effective stress engendered by the pore pressure decline and associated reservoir compaction is considered as the main reason for reactivation of faults. Herein, we use hydromechanically coupled finite element models to investigate how and when a critical state of stress emerges on a fault during pore pressure decline for a setting typical for the North German Basin. Proceeding from this critical state of stress and assuming a velocity weakening friction law the dynamic rupture is modelled. Normal faulting slip is found at the depth of the reservoir nucleating near its top in the hanging wall. A vertical offset of the reservoir at the fault of half its thickness produces the largest earthquake among the cases tested between zero offset and equal to the reservoir thickness, but fault reactivation occurs first for larger offsets. Regarding the relative position of production wells to the fault simultaneous production from both compartments leads to earlier reactivation and larger earthquakes compared to if production is from only one compartment. The frictional properties of the fault are found to have the largest impact on coseismic displacement and stress drop. A crucial role on the timing of rupture initiation and size of the rupture can be attributed to the pre-production stress state.